1. Field of the Invention
The present invention relates to a process for the production of an aromatic polycarbonate. More specifically, it relates to a process for the production of a polycarbonate which is almost free from being colored, which is also almost free from a decrease in polymerization degree when molded, and which is further almost free from forming branches and insolubles.
2. Description of the Prior Art
A polycarbonate is widely used, since it is excellent in mechanical properties such as impact resistance and is also excellent in heat resistance and transparency. For producing this polycarbonate, there is known a method in which an aromatic dihydroxy compound such as bisphenol and phosgene are directly reacted (interfacial method) or a method in which an aromatic dihydroxy compound such as bisphenol and a diaryl carbonate such as diphenyl carbonate are reacted in an ester exchange reaction in a molten state (melt method).
Of the above methods, the method of producing a polycarbonate by reacting an aromatic dihydroxy compound and a diaryl carbonate in an ester exchange reaction is carried out as follows. In the presence of a catalyst selected from organic acid salts, inorganic acid salts, oxides, hydroxides, hydrides and alcoholates of metals, an aromatic dihydroxy compound and a diaryl carbonate are reacted in an ester exchange reaction while they are melted by heating them, for example, up to a final temperature of 250.degree. to 330.degree. C. under reduced pressure.
This method (melt method) is advantageous over the above interfacial method in that a polycarbonate can be produced at a low cost. However, the problem with this method is that the so-obtained polycarbonate has a poor hue since the polymer is exposed to a high temperature for a long period of reaction time.
JP-A-62-199618 (the term "JP-A" as used herein means an "unexamined published Japanese patent application") discloses a method of producing a polyester carbonate, in which a diaryl carbonate and an aromatic dicarboxylic acid alkyl ester are reacted in an ester exchange reaction in the presence of a catalyst while they are melted.
As the catalyst, the above JP-A-62-199618 describes (1) alkali metals and compounds thereof, (2) compounds of elements belonging to the Groups II (group 2) and III (group 13) of the periodic table and (3) compounds of metals other than the above, such as germanium, tin, lead, zinc and cadmium. Examples of JP-A-62-199618 only disclose methods of producing polyester carbonates in the presence of a tin compound alone.
JP-A-54-63023 discloses a method of reacting a hydroxy compound and carbonate in an ester exchange reaction in the presence of a tin alkoxide of the formula, EQU R.sub.3-k Sn(OR.sup.1).sub.1+k
wherein R is a hydrocarbon residue, R.sup.1 is a hydrocarbon residue and k is an integer of 0 to 2, provided that two R.sup.1 s may represent one alkylene group. PA1 wherein Y is O--CO--R.sup.2, OH or OR.sup.2 group in which R.sup.2 is C.sub.1-12 alkyl, C.sub.5-12 aryl or C.sub.7-13 aralkyl, R.sup.1 has the same meaning as that of R.sup.2, and x is an integer of 1 to 3, di(C.sub.1-12 alkyl)SnO and a compound of the following formula, ##STR1## wherein R.sup.4 is has the same meaning as that of R.sup.2, and R.sup.5 either has the same meaning as that of R.sup.2 or represents an OR.sup.2 group. PA1 wherein each of R and R.sup.1 is independently a C.sub.1-20 hydrocarbon group and n is an integer of 0, 1, 2 or 3. PA1 (a) an alkali metal salt of ate-complex of a metal element of the group 14 of the periodic table, or PA1 (b) an alkali metal salt of oxoacid of a metal element of the group 14 of the periodic table. PA1 wherein M.sup.1 is an alkali metal, M.sup.2 is silicon, germanium, stannum (tin) or plumbum (lead), X.sup.1 is an alkyl group having 1 to 20 carbon atoms, a cycloalkyl group having 5 to 20 carbon atoms or an aryl group having 6 to 20 carbon atoms, R.sup.6 is an alkyl group having 1 to 20 carbon atoms, a cycloalkyl group having 5 to 20 carbon atoms or an aryl group having 6 to 20 carbon atoms, and each of p and q is an integer of 0 or 1 to 5, provided that p+q equals 3 or 5. PA1 wherein M.sup.1, M.sup.2 and R.sup.6 are as defined in the formula (II), and r is 3 or 5. PA1 wherein M.sup.1, M.sup.2 and X.sup.1 are as defined in the formula (II), and s is 3 or 5. PA1 wherein M.sup.1, M.sup.2, X.sup.1 and R.sup.6 are as defined in the formula (II), and each of t and u is an integer of 1 to 4 provided that t+u equals 3 or 5. PA1 wherein each of R.sup.7, R.sup.8, R.sup.9 and R.sup.10 is independently an alkyl group having 1 to 20 carbon atoms or a cycloalkyl, cycloalkylalkyl, aryl or aralkyl group having 6 to 20 carbon atoms, and X.sup.2 is OR.sup.11, --OCOR.sup.12, B(R.sup.13).sub.4 or F in which R.sup.11 is a hydrogen atom, an alkyl group having 1 to 20 carbon atoms or a cycloalkyl, cycloalkylalkyl, aryl or aralkyl group having 6 to 20 carbon atoms, R.sup.12 is a hydrogen atom, an alkyl group having 1 to 20 carbon atoms or a cycloalkyl, cycloalkylalkyl, aryl or aralkyl group having 6 to 20 carbon atoms and R.sup.13 is a hydrogen atom or an aryl group having 6 to 20 carbon atoms. PA1 wherein R.sup.7, R.sup.8, R.sup.9, R.sup.10 and R.sup.11 are as defined in the formula (III). PA1 wherein R.sup.7, R.sup.8, R.sup.9, R.sup.10 and R.sup.12 are as defined in the formula (III). PA1 wherein R.sup.7, R.sup.8, R.sup.9, R.sup.10 and R.sup.13 are as defined in the formula (III). PA1 wherein R.sup.7, R.sup.8, R.sup.9 and R.sup.10 are as defined in the formula (III).
As the tin alkoxide, the above JP-A-54-63023 describes tin trialkylalkoxide, tin dialkyldialkoxide, tin diaryldialkoxide and tin alkyltrialkoxide.
JP-A-57-2334 discloses a method of producing an aromatic polycarbonate, in which a dihydric phenol and dialkyl carboxylate are reacted in an ester exchange reaction in the presence of an organic tin (IV) compound (a) and a monohydric phenol (b).
The above organic tin (IV) compound includes a compound of the following formula, EQU (R.sup.1).sub.4-x Sn(Y).sub.x
Examples of JP-A-57-2234 use ethyl tin triisooctylate and ethyl tin tributoxide.
JP-A-6-145334 discloses a method of producing a polycarbonate, in which an aromatic diol compound and a carbonic acid diester are melt-polycondensed in the presence of an organic silicon compound of the formula, EQU R.sub.n Si(OR.sup.1).sub.4-n
As an ester exchange catalyst used for the above melt-polycondensation, JP-A-6-145334 discloses acetates, carbonates, borates, nitrates, oxides, hydroxides, hydrides and alcoholates of metals such as alkali metals, alkaline earth metals, tin, nickel and titanium. In Examples, JP-A-6-145334 uses dibutyl tin oxide (Examples 1-8) and dibutyl tin dilaurate (Example 9).
JP-A-5-202180 discloses linear polycarbonates having a branch parameter G of 0.8 to 1.0. The branch parameter G is defined as a value obtained by dividing an intrinsic viscosity [.eta.] in methylene chloride at 20.degree. C. by the intrinsic viscosity [.eta.].sub.lin measured in methylene chloride at 20.degree. C., of a linear polycarbonate having the same average molecular weight measured by a light scattering method. It can be therefore said that the smaller the value G is, the greater the branching degree is.
Concerning alkali metal compounds and alkaline earth metal compounds known as an ester exchange catalyst, JP-A-5-202180 describes that these compounds have high activity as a catalyst but that, disadvantageously, a branched structure is formed by side reactions, a polycarbonate is partially insoluble in methylene chloride or a polycarbonate is colored to excess.
JP-A-5-202180 therefore discloses that the catalyst can be selected from compounds containing elements of Group IIb (group 12) of the periodic table such as zinc and cadmium, compounds containing elements of Group IVb (group 14) of the periodic table such as silicon, germanium, tin and lead and compounds containing elements of Group Vb (group 15) of the periodic table such as antimony and bismuth. Specifically, JP-A-5-202180 describes silicon oxide. germanium oxide, germanium hydroxide and tin (IV) oxide.